It looks like your gateways are being deselected on grounds of not having them set up properly (disabled interfaces or being marked as inactive because an interface may be missing). Please double-check your missing gateways setup.

However, I've added a patch that makes all gateways show up in the rules edit page, that should fix your visibility issue and makes all gateways selectable, but your setup may still be off. You can import the fix from the console:

I can't seem to reproduce this. I add a gateway group, it gets added in the firewall rule drop down. I add another one, it gets added to the dropdown, too.

I've asked Boris for box access. These issues are trivial to fix, but we need the exact conditions to reproduce or the actual box where this happens in order to go forward.

There is a small GUI issue where the last entry of a drop-down is truncated a bit, but the typeahead still works and looking at the attached screenshot it's still possible to select the last gateway. (Just in case this is the reported issue, which is annoying but not a high-priority item.)